This invention relates to solenoidal electric field (SEF) lamps, and in particular to circuitry for initiating lamp operation.
U.S. Pat. No. 4,005,330 to Homer H. Glascock, Jr. and John M. Anderson and U.S. Pat. No. 4,017,764 to John M. Anderson describe a class of induction ionized flourescent lamps wherein a high frequency, solenoidal electric field is established by a lamp core having a torodial shape which is centrally disposed with respect to a substantially globular envelope. The lamps described in these patents may be manufactured in a form which is electrically and mechanically compatible with the common Edison base incandescent lamp and which provides substantially more efficient operation than conventional incandescent lamps. The above Glascock and Anderson patents are hereby incorporated herein as background material.
In such SEF lamps, an annular core typically comprising ferrite is disposed within or about an ionizable gas such as mercury vapor. This annular core possesses an electrical winding for coupling to a radio frequency energy source. The electrical energy being supplied to this core winding creates a solenoidal electric field within the ionizable medium of sufficient strength to produce current flow in the plasma, once plasma ionization occurs. The plasma ionization and subsequent current flow produces electromagnetic radiation at a first frequency through electron transition in the medium. Typically, when the ionizable medium comprises mercury as a major portion, the electromagnetic radiation lies in the ultraviolet region of the spectrum. In the typical case, ultraviolet radiation per se is not the optical output desired and the envelope containing the ionizable medium is conventionally coated with a phosphor which absorbs energy at the first frequency and reradiates electromagnetic energy at a second, optical frequency or frequencies depending upon the combination of phosphors employed.
The SEF lamp has two major portions associated therewith. First there is the envelope portion itself typically comprising an envelope, one or more toroidal ferrite cores with windings thereon and an ionizable fill gas contained in the envelope which typically possesses an internal phosphor coating. The SEF lamp also comprises a ballast portion which operates to convert conventional line current to higher frequency voltage pulses which are more efficient for lamp operation. Push-pull inverter circuits with appropriate control modalities are particularly useful for supplying the desired voltage pulses. Because the ionizable medium has a negative resistance characteristic, it is necessary to electrically couple the core winding to the ballast circuit through one or more ballast reactances to limit the current flow following plasma ionization during which the effective resistance of the plasma decreases. Thus, the lamp core operates in a transformer, the primary winding of which being the core winding connected to the ballast circuit, the secondary of which being the single turn of current flow through the plasma along the lines of the solenodial electric field.
Before the lamp enters into the negative resistance portion of its operating curve, it is first necessary to initially ionize a portion of the plasma to effect easy lamp starting. While it is possible to effect lamp starting simply by providing greater energy input into the core winding in a short period of time, this method of lamp starting is undesirable since it produces an unnecessary level of core heating thereby increasing the possibility that the Curie temperature of the ferrite core is exceeded and this method also results in undesirable levels of noise from the lamp components. Another method of accomplishing lamp starting is to dispose an additional winding or windings on the lamp core. The starting winding on the core may comprise a second separate winding, but this is not preferred. Alternately, the start winding may be disposed on the core and configured with the primary winding on the core so as to operate as an autotransformer as disclosed in application Ser. No. 799,300 filed May 23, 1977 in the name of Loren H. Walker and the inventor herein which invention is assigned to the same assignee as the present invention. However, because of the relatively high temperature at which the core operates, particularly in an SEF lamp configuration in which the core is disposed within the ionizable medium itself, it is necessary to provide expensive high temperature insulation for the additional turns required on the core.